Grain refinement process for copper-bismuth alloys



United States Patent U.S. Cl. 148-2 4 Claims ABSTRACT OF THE DISCLOSURECopper-bismuth alloys with a refined grain structure are prepared bycasting the alloys, which contain from 0.05% to 1% of bismuth, into theform of ingots, heating the ingots to temperatures in the range of 300C. to 950 C., and extruding the heated ingots at an extrusion ratio suchthat the alloy is maintained in a state of hydrostatic compressionduring deformation, and that tensile forces are absent, a preferredextrusion ratio being 4:1.

This invention is concerned with a method of improving the properties ofcopper-bismuth alloys intended for use in electrical devices, and isparticularly concerned with a process for preparing such alloys with arefined grain structure.

An alloy with anti-welding properties is required at the contact facesof vacuum switches. A copper-bismuth alloy is favoured for this purpose,the alloy usually being produced in the cast state by melting processes.A drawback of the cast material is the inherently large grain size whichmay lead to the plucking of pieces of material, corresponding to thelarge grains, from the contact faces.Such pieces of material may causerestriking, or obstruct reclosure of the switch. This difficulty may beovercome by reducing the grain size of the alloy. The usual methods ofgrain refinement by forging or rolling are not possible withcopper-bismuth alloys due to the brittle nature of the material.

This invention consists in a method of preparing a copper-bismuth alloywith a refined grain structure, which comprises subjecting the alloy toan extrusion process at a temperature in the range 300-950 C., theextrusion ratio being typically 4:1. It is preferable to extrude at atemperature in the lower part of the range.

An important feature of the extrusion process is that the alloy ismaintained in a state of hydrostatic compression during deformation, andthat tensile forces are absent. Intergranular cracking which normallytakes place via the bismuth phase in the alloy is thereby avoided.

The invention is put into practice in the following way. Copper-bismuthalloys in the range 0.051% bismuth by weight are melted and cast intoingots by a suitable technique. One such technique, which has beenemployed for vacuum switch alloy production, involves high frequencymelting in a graphite crucible in a vacuum. In this technique it isnecessary to alloy for any loss of the more volatile element bismuththat occurs, to ensure that the desired alloy composition is obtained.The ingots from the casting operation are machined to produce billetsappropriate for extrusion. Typical billets would be cylindrical, 4"diameter x 6" long. At this stage and before extrusion it may bedesirable to enclose the alloy billet in a ductile can. The can serves adual purpose, firstly to give support to the alloy during the extrusionprocess and also to prevent the pick up of gaseous or other contaminantswhich are deleterious to the satisfactory functioning of a vacuumswitch. It has been found that copper tube forms a suitable canningmaterial for the alloy billet, the ends of the tube being sealed bywelded-in copper plugs. The pick-up of gaseous contaminants can befurther reduced by evacuating the can before sealing.

For extrusion, the billet is heated by any appropriate means to theextrusion temperature, which for copper-bismuth alloys lies in the range300-950 C. The heated billet is extruded through a die of the requireddimensions. If a cylindrical rod is required, this may be produced byextrusion through a circular die. Alternatively, strip may be obtainedby extrusion through a die of rectangular cross section. Various othershapes of extruded product are also possible.

The canning material extrudes simultaneously with the alloy, maintaininga gas tight sheath on the alloy. The sheath is removed by machining.

The magnitude of the grain refining effect is shown by the followingexamples.

Example 1 An alloy cast from copper with 0.5% added bismuth was machinedto give a 2" diameter extrusion billets. This was heated to 850 C. andextruded to 1 diameter, i.e., with 4:1 extrusion ratio. The cast grainsize of 2-3 mm. was reduced to a fine equiaxed structure with grains mm.diameter by this method.

Example 2 An alloy of copper with 0.5 bismuth by weight was extruded at500 C. with a 4:1 extrusion ratio, viz. from a 2" diameter billet to 1"diameter. After extrusion the structure showed elongated grains ofcopper interspersed with bismuth, demonstrating that at this extrusiontemperature the copper retains a worked structure. On annealing thealloy at 800 C. for 1 hour to simulate processing on a vacuum switch thecopper phase was annealed and an equiaxed grain structure was producedwith grain size 0.2-0.3 mm. compared with the cast grain size of 2-3 mm.

What we claim is:

1. A method of manufacturing a copper-bismuth alloy with a refined grainstructure, which includes the steps of casting a copper-bismuth alloycontaining 0.05% to 1% bismuth into the form of an ingot, heating theingot to a temperature in the range of 300 C. to 950 C., and extrudingthe heated ingot at an extrusion ratio such that the alloy is maintainedin a state of hydrostatic compression during deformation and thattensile forces are absent.

2. A method according to claim 1, in which the ingot is machined to theform of a cylindrical billet which is 3 then enclosed in a copper tube,the ends of the tube being sealed by welded copper plugs, and the billetis then subjected to the extrusion process, the copper covering beinglater removed by machining.

3. A method according to claim 1, in which a copperbismuth alloycontaining 0.5% bismuth is extruded at a temperature of 850 C., theextrusion ratio being 4:1.

4. A method according to claim 1, in which a copperbismuth alloycontaining 0.5% bismuth is extruded at 500 C., the extrusion ratio being4:1, and the extruded alloy is thereafter subjected to annealing bybeing heated to 800 C. for one hour.

References Cited UNITED STATES PATENTS 4 2,975,256 3/1961 Lee et a1200-166 X 3,122,828 3/ 1964 Havel 29-528 X 3,246,979 4/ 1966 Laiferty eta1 75-153 X 5 OTHER REFERENCES Metals, Carpenter and Robertson, OxfordUniv. Press, New York, 1939, relied on pp. 1255-1257.

The Journal of the Institute of Metals, vol. 37, No. 1, 1 1927, reliedon pp. 169-478.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R.

